Conventional techniques for extracting rare earths from monazite and bastnasite include caustic soda decomposition and sulfuric acid roasting. In caustic soda decomposition, after pretreatment with NaOH, rare earths are converted into rare earth trihydroxides (RE(OH)3), which can be leached with mineral acids to solubilize the rare earths into a leachate; this process requires large amounts of NaOH and increases the difficulty of subsequent processing steps. In sulfuric acid roasting, the roasted rare earth concentrate with H2SO4 is leached with water to recover rare earths into a leachate and separated from the gangue components; although a fraction of the fluorine can be removed from the gas stream, rare earths are difficult to recover from the aqueous leachate due to the interaction of rare earths with the remaining fluorine. If the fluorine-containing ore or concentrate is treated with the technique of sulfuric acid roasting, although a fraction of the fluorine can be removed from the gas stream, rare earths are difficult to sufficiently recover from the aqueous leachate due to the interaction of rare earths with the remaining fluorine.
It has been proved experimentally that the removal of fluorine by mineral processing techniques is very difficult. Moreover, rare earth ores or concentrates may also contain other metals of interest, such as, by way of non-limiting example, niobium, tantalum, zirconium, hafnium, and titanium, which are not effectively recovered and separated by caustic soda decomposition or sulfuric acid roasting.
Chlorination is a suitable technique for processing ores or concentrates that contain both rare earths and other metals of interest. One of the most straightforward processes known in the art for recovering rare earths and the like from raw ores or concentrates may be termed “direct” carbochlorination, i.e. carbochlorination of the ore or concentrate without any additives or pretreatments. In direct carbochlorination, rare earths are converted into their chlorides and enriched in a solid or molten calcine, while other metals of interest form volatile chlorides in a gaseous phase; for ores or concentrates with low concentrations of rare earths, the carbochlorination calcine is then leached, and the rare earth chlorides can be recovered, separated, and purified in subsequent hydrometallurgical treatment of the leachate.
The following four references generally relate to direct carbochlorination processes and are incorporated herein by reference in their entireties:
F. R. Hartley, “The preparation of anhydrous lanthanon chlorides by high-temperature chlorination of monazite,” 2(1) Journal of Applied Chemistry 24 (January 1952).
A. W. Henderson et al., “Chlorination of euxenite concentrates,” 50(4) Industrial & Engineering Chemistry 611 (April 1958).
O. M. Hilal and F. A. El Gohary, “Chlorination of monazite,” 53(12) Industrial & Engineering Chemistry 997 (December 1961).
W. Brugger and E. Greinacher, “A process for direct chlorination of rare earth ores at high temperatures on a production scale,” 19(12) Journal of Metals 32 (December 1967).
However, to effectively recover rare earths from the carbochlorination calcine with dilute hydrochloric acid leaching, all of the rare earths should be in the state of their chlorides, and the formation of rare earth fluorides must be avoided because rare earth fluorides are insoluble in dilute hydrochloric acid. Thermodynamically, the rare earth fluorides are more stable than their chlorides, and when there is fluorine in the ore or concentrate, the formation of rare earth fluorides during direct carbochlorination is thus unavoidable. As a result, the rare earths mainly remain in the solid residue after leaching the chlorinated materials, and so recovery of rare earths from the leachate after direct carbochlorination is not feasible when the ore or concentrate contains appreciable fluorine content.
In addition, in the techniques disclosed in the prior art, such as U.S. Pat. No. 3,353,928 to Woyski et al. (“Woyski,” the entirety of which is incorporated herein by reference), it is necessary to remove fluorine from the system by forming volatile fluorides.
There is thus a need for a method of recovering rare earths and the like, which retains the benefits of direct carbochlorination but results in high yields of the rare earths from raw ores or concentrates which contain fluorine. There is a further need for such methods that do not require the complete removal of fluorine from the system.